We have employed near infrared spectroscopy to quantify each of the compounds ATP,ADP, and PO 4. We have successfully made calibration curves for each in the 1 to 25 micromolar regime. Recently, mixtures of the three phosphate compounds were also quantified in the same concentration region. Hence, the distinction of ATP, ADP, and P(i) can be achieved with near infrared. Because there are significant variations in the near infrared spectra of ATP and ADP and the majority of change between ATP and ADP lies within the phosphate moiety, it follows that the differences being followed in the near infrared spectra of ADP and ATP lie within this environment.